1. Field of the Invention
The invention relates to a vibration damping device and a power transmission device, and more particularly, to a vibration damping device that damps vibrations of a rotary shaft in a translational direction other than a rotational direction thereof using an inertial mass (mass body), and a power transmission device fitted with the vibration damping device.
2. Description of the Related Art
As a vibration damping device that exerts a vibration damping effect in the vicinity of a resonance frequency of a vibration system, a dynamic vibration absorber composed of an inertial mass and an elastic body is often employed. For example, there is known a vibration damping device with an inertial mass and an elastic body whose rigidities are so tuned as to exert a vibration damping effect at the time of bending resonance or torsional resonance of a rotary shaft in a power transmission path of a vehicle.
In such a damper device, however, the rigidity of the elastic body changes due to a change in the temperature of an environment in which the damper device is used, and the rotational speed of the rotary shaft is likely to fluctuate due to a shift of the center of gravity of the inertial mass. In some cases, therefore, it is difficult to obtain a stable vibration damping effect.
Thus, there is a damper weight obtained by, for example, mounting an automatic aligning bearing on a rotary shaft and providing an outer race thereof with a mass (e.g., see Japanese Patent Application Publication No. 10-306844 (JP-A-10-306844)).
However, in the vibration damping device according to the related art as described above, the inertial mass body is rockable on a plane including a rotational center axis of the rotary shaft. Therefore, there is a problem in that bending vibrations of the rotary shaft cannot be effectively damped.
Further, among various noises/vibrations of a rear-wheel-drive vehicle, there is a whining noise on a vehicle interior side resulting from the resonance of a rotation input portion of a rear differential device as a phenomenon with a high degree of difficulty which causes troubles in ensuring the performance. As a measure against this whining noise, it is conceivable to fit the rotation input portion of the rear differential device with a torsional damper so as to suppress a compelling force causing resonance of the rotation input portion. However, the torsional damper is more costly than a mass damper. Besides, in the torsional damper, a resonance frequency exerting a vibration damping effect for torsional resonance is often greatly different from a resonance frequency exerting a vibration damping effect for bending resonance. It is very difficult to achieve both the vibration damping effects for torsional resonance and bending resonance, whose resonance frequencies are greatly different from each other, within a limited installation space, namely, the input portion of the differential device. Thus, in the case where torsional resonance is sufficiently damped at a certain frequency to attempt a reduction in the level of vibrations, the vibration level of bending resonance may worsen at other frequencies.
In addition, the torsional damper achieves a vibration damping effect by adding a new vibration system to existing torsional vibrations and hence changing the compliance characteristic of the torsional vibrations. For structural reasons, however, an inertial mass acts in a rotational direction of the existing torsional vibrations. Therefore, the peak level of the torsional vibrations tend to decrease, and a mesh point dynamic rigidity (compelling force) emerging as an inverse of compliance tends to worsen. Furthermore, around a target frequency, there always emerges a frequency lowering the level of vibrations because of the formation of a trough in the compliance characteristic. The mesh point dynamic rigidity is likely to worsen in that respect as well. As described above, the torsional damper has both an effect of reducing the level of vibrations for a compelling force and an adverse influence accompanied thereby. In some cases, therefore, a sufficient vibration damping effect as expected cannot be exerted even when the characteristic of the damper is adjusted.